The present invention relates to tomography systems and operating methods and more particularly to systems and methods for implementing helical computerized tomography.
Computerized tomography (CT) is a well established technology for imaging cross-sectional planes of objects nondestructively.
CT systems generally may direct source electromagnetic radiation through an object and detect the radiation altered by the object to provide information about the nature of the object. Normally, a CT system comprises a source of X-radiation, an array of X-ray detectors, a mechanism for rotating the source and detectors continuously around a reference axis extending through the object, a couch for advancing the object continuously along the reference axis, data collection hardware for converting detector signals to digital records, data processing hardware for reconstructing a plurality of two-dimensional images from the digital records, and imaging hardware for displaying and recording the images.
In CT systems designed to generate planar scans, X-ray slice data is collected by a planar array of detectors placed opposite an X-ray source, both of which rotate rigidly in a plane around a stationary object. Alternatively, the X-ray source and the detectors may remain stationary while the object rotates about its own axis.
Standard planar scan CT systems collect a plurality of two-dimensional cross-sectional planes (slices) by rotating the radiation source and one linear array, composed of discrete detector elements, through either 180.degree. or 360.degree. (depending upon the geometry of the source/detector hardware). The rotation is then stopped and the object is advanced parallel to the axis of rotation, where the operation is repeated. The thickness of such a CT slice is determined by the thicknesses of the radiation source and the detectors. To obtain continuous slices, the object is advanced at most by a distance equal to the beam thickness.
The data obtained for each slice by a 360.degree. rotation is processed by a mathematical reconstruction algorithm to produce a two-dimensional image of the slice. A 360.degree. rotation provides more data than the minimum required data set (180.degree. rotation plus the angle of the X-ray fan), but the redundant data is useful since it minimizes inconsistencies between readings taken at the beginning and at the end of the scan. The incremental planar scan procedure produces good images but it is time-consuming.
Data has been collected faster and more efficiently by continuously advancing the object along its axis while the source and detectors also rotate continuously. CT systems for which the object is incremented continuously are typically called "helical" CT systems because the motion of the radiation source as a function of time follows a helical path with respect to a coordinate system fixed in the object being imaged. Data collection speed is maximized if the object is advanced continuously, but the total increment must be much less than the nominal beam thickness if relatively artifact-free images are to be obtained.
The resulting helical data set is less than ideal because data fans taken 360.degree. apart do not lie in the same plane, but if the translational distance is comparable to the slice width of the X-ray beam, satisfactory images can be constructed. Even though single-detector-array, helical scans are faster than planar scans, they are still too slow for many purposes. Thus, as one particularly important example, the fastest known helical CT systems, designed for use in medical tomography, have been too slow to provide data collection for a significant volume of the human chest during a single breath hold, i.e. about 40 seconds.